To determine whether there is a trend toward using the summer session as a fourth quarter, the enrollment patterns of 3 groups of students at the University of Minnesota were compared over a 5-year period covering the 1964 to 1968 summer sessions and the 1963-1964 to 1967-1968 academic years. The student groups comprised degree candidates who attended the university only during the academic years, others who studied only during the summers, and "summer session only" students who registered during 1 or several summers but were not degree candidates. No established year-to-year enrollment pattern was found over a 5-year span within summer sessions or academic years or between the 2 registration periods, but similar enrollment increases were revealed. The proportion of "summer session only" students decreased by more than 5% between 1964 and 1968, and the proportion of academic-year students who attended summer sessions increased by more than 5%. These proportions reflect that the number of academic-year students within the summer sessions has increased at a faster rate than the number of "summer session only" students, but do not indicate a changing trend toward a four-quarter system. The most significant finding of the study is the stability of relationships of enrollment patterns revealed by the comparison between the summer sessions and the academic years. (WM)